The focus of this laboratory in the last few years has been the structural characterization of clathrin and clathrin coated vesicles that are involved in receptor-mediated endocytosis. Continuing our work on these lines, we have identified a new polymer of clathrin that is the smallest reported hitherto under specific buffer conditions namely 2mM MES, pH 5.9. This polymer has a sedimentation coefficient of 27S. Deep etch electron microscopy showed that this species is formed from clathrin triskelions (8S) with each triskelion extending its leg to join the globular end of the other triskelion. The extended nature of this triskelion leg is supported independently by Pearse's group at MRC, England, where they have shown that clathrin can form a cube like structure with a sedimentation coefficient of 42S. Equilibrium centrifugation of the 27S in 3M glycerol yielded a molecular weight of the polymer corresponding to four triskelions independently confirming the electron microscopic data.